1. Field of the Invention
This invention relates to a method of rendering an ink strippable and more particularly, to rendering a pattern of an oil-modified alkyd ink or a drying oil vehicle ink strippable from a surface upon which it is applied and dried.
2. Discussion of the Prior Art
A method for fabricating printed wiring boards involves the so-called "subtractive" technique. In the subtractive technique a laminate comprising metal sheets or laminae, e.g., copper laminae, adherently deposited or held on opposed surfaces of a dielectric substrate is selected for treatment. At least one of the metal laminae is selectively coated with an etch resistant material to form a masking pattern. The resultant masked surface is then subjected to an etchant whereby the unmasked portions of the metal layer are removed by etching. The masking pattern is then removed to yield a conductive pattern remaining on the dielectric substrate.
In practice, the etch resistant material typically comprises a photoresist which has to be initially selectively exposed to a source of light energy, followed by solvent development to form the masking pattern. Utilizing such a photoresist material is both time consuming and expensive. Alternatively, etch-resistant inks with solvent vehicles have been utilized as masking materials. These inks are screen printed on the metal surface and then dried by heating the surface and driving off the solvents employed. The problems with using such materials are that screen printing is a slower process than other printing techniques, such as offset printing, line widths under 20 mils are generally not feasible and also pollution is involved whenever solvents have to be driven off from an applied ink.
Offset printing techniques have been utilized to form the masking pattern. However, difficulties are encountered in finding inks which are both suitable for printing techniques, such as offset printing, and which can be easily or practically removed from the metal surface after serving as an etch resist. Oftentimes the ink or resist pattern cannot be easily removed, as by treatment with a solvent, and must be removed by such expedients as mechanical grinding. U.S. Pat. No. 2,602,731 reveals such an expedient.
A group of inks which are suitable for offset printing comprises resins which are known as oil-modified alkyd resins. An oil-modified alkyd resin comprises the reaction product of a dibasic (difunctional) acid or acid anhydride, e.g., phthalic anhydride, a polyol, e.g., glycerin, and a drying oil fatty acid, e.g., levulinic acid. Another group of inks which are suitable for offset printing comprises resins or vehicles known as drying oils. These inks are formulated and manufactured from such typical oils as linseed, tung, soybean, safflower, dehydrated castor, poppyseed, oiticia and perilla oils. Both groups of inks, upon drying, that is upon polymerizing and/or crosslinking, form a film which resists scuffing and abrasion and because of this, find wide utility in decorating metal surfaces. However, these alkyd containing and drying oil containing inks, upon drying, are nearly impossible to remove by solvent stripping. Such alkyd resin and drying oil containing inks are not therefore being used for offset printing etch resist patterns in subtractive fabrication of printed wiring boards.
A method of rendering an oil-modified alkyd resin ink pattern and a drying oil vehicle ink pattern capable of being readily solvent stripped from a surface to which it is applied is needed and is desired, especially for offset printing of resist patterns in subtractive printed wiring board manufacture.